openai/gpt-chat-latest → B (4:1)

Side B adds enduring functionality: a persistent theme system (theme cookie handling, `/theme` endpoint, layout updates, propagation through auth and page rendering) and correct room-aware web/API URLs via `item_path_for_api_in_room` and `forum_thread_web_url`, with accompanying tests and widespread RPC fixes. Side A mainly removes the obsolete monolithic `server/src/html/forum.rs` and establishes `forum/mod.rs` as the sole module root, a valuable cleanup that resolves module ambiguity but largely reorganizes existing code rather than adding new behavior.